Material drying method



May 3o, 1944.

C. W. GORDON MATERIAL DRYING METHOD Filed April 24, 1941 @H4/ems' M Gaim/V.

l BYM l particles.

Passau-May so, 1944 UNITED sTATEs PATENT oF-FICE 2,350,162 t MATERIAL parme. ME'rnoD Charles W. Go'rdo'n, Glen Ellyn, ill., assignorv to Combustion Engineering Company, Inc., New

York. N. Y.

Application April 24, 1941', serial No. 390,063

s claims. (ci. 34--1oi This invention relates to drying moisture laden materials and particularly to an improved method for drying sewage sludge so as to control the particle size of the product. v-

ln "drying sewage sludge by a flash drying process such as disclosed in the U. S. Patent No.

2,157,775 granted May 9, 1939', to Alfred R. Smith, the dried product has consisted of line sized particles suitable for use as fuel in the furnace from which` weretaken the hot gases for the drying process. The ilneness of the dried'product while desirable for burning it in suspension'is disadvantageous for handlingvand for storage because of the lire risk. However, there is a market for a coarser `dried product suitable for use as fertilizer. It )is an object of this invention to control the operation of a filash drying system so that the dried product delivered from the system may be of predetermined particle sizes.

In the accompanying drawing the single ligure is a diagrammatic view of a flash drying system suited to carrying out the invention.

y Raw wet material is fed into a hopper I andl delivered to a mixer I2 having a mixing screw I4.

Previously dried material, known as dry return,

` is fed to the mixer through a pipe I6 and the raw material and dry return become intimately mixed during their progress through the mixing chamber so that'by the time the mixture reaches the outlet I8 it consists ofa mass of small discrete through outlet' I8 to a drying tower 20 where it enters a current of hot gases whichin the par- This mass of separate particles is fed.

the second one. open and then completely close ticuiar form shown arethe products of combus` i tion from a furnace 22. f The current of hot gases causes a portion of the moisture in the particles to flash into steam. The evaporating or drying process is continu in the dash drier 24, in which there is a rotor from tower 20 through `the central opening 4l. After being thoroughly agitated within the curdrier, the mixture of gases, evaporated moisture and drie' particlesleaves the drier 24'and'passes rent of'liot gases while passing through the flash the second branch, leaving the rst branch open. Any changes in the ratio of distribution between the two branches may be made by adjusting the length of time which one of the branches is open relatively to the length of time the other is open. lFor example, if it is found that for proper operation two thirds of the dried material should be returned and one third discharged, the passage II is kept open two umts of time for each unit of time that the branch 5I is open.

The drying system described above is essentially that of the Smith patent. Utilizing drying apparatus of this type I have vfound that by regulating the moisture content 0f the mixture of material delivered to drier 24, varying the speed of ,operation of the mixer I2 and drier 24,

, and controlling the temperature in the cyclone 46, I may obtain a final product of a desired particle size. lI have found that the particle size tends to increase with an increase of moisture in` the sludge delivered by' the mixer. This may be simply controlled by the timing of the damper 52 to deliver less of the dry material to the mixer.

I have also yifound that decreasing the speed of drier 24 substantially below present practice tends to enlarge the particles. This may be controlled by adjusting the speed of motor 28. I have further found thatdecreasing thetempe'rature existing in the cyclone considerably below those presently employed causes the grain size to increase. This may be effected by decreasing the amount of heat delivered to the system byfur- Qnace 22. 'Ihis last measureof control will also lower the degree of drying of the material. Accordingly where an exceptionally large 4particle size is required resulting in a product that is' excessively moist, I may employ a second stage of drying to complete evaporation of moisture. s My novel method of drying for controlled particle Size may beVV carried out within the approxlf mate ranges of drying conditions .as follows:

In usual ilash drying the tip speed ofthe `outer rotating bars of drier 24 is about 5000 teet'v mi: uta. When employing my invention this speed normaiiyvaries feet per nte.

In usual ilash drying, moisture content of the mixture yvaries between 25% to 35% while I increase the moisture content to between 35% and y speedoi `3200 'feet per minute and maintaining a temperature of 300 deg. F. in the cyclone separator.

What I claim is:

l. In the art oi' drying moisture laden sewage sludge materials in a ydrying system wherein the material to be dried is-carrieci in a stream of hot gas through a rotary drier oi' the beater type afrom which the material and gas pass to a sep-,

arator, the improved method which comprises;

mixing previously dried sludge with the fresh 'sludge in proportions tor maintaining the moisturecontent of the material to be dried at be- -tween 35% to 50%, mixing hot gases with the sludge before admitting the mixture Vto the dryer, operatingvthe drier at a peripheral speed of between 2000 to 4000 reet per minute for the tips In usual ilash drying the temperature exist- 1 which the material and aac-enea oi the drier ters, and inaintaimingv a tern-x peinture of between 200 deg. F. to 350 deg. F. in the separater.

2. In the ari; oi g moisture laden sewage sludge materials in a drying system wherein the material to be dried is carried in a stream oi hot gas through a rotary xdrier of the beater type fram gas pass to a separater, the improved method which comprises; mixing previously dried sludge with the ufresh sludge in proportions for maintaining the moisture content of the material to be dried at anproximateiy 40%, mixing hot gases with the sludge before admitting the mixture to the dryer, operating the drier at a peripheralV speed of approximately 3200 feet per minute for beaters. and maintaining a temperature oi 300 deg. F, in the separator.

3. In the artoi drying sewage sludge ha a moisture content of to 85 per cent in a drying system wherein the stream ot hotv gas through a rotary drier oi the beater type from which the material and gas pass to a separator, the improved method which comprises: mixing previously dried material with the fresh sludge in proportions to maintain the moisture content of the sludge Vto be dried-approximately'40 per ent, mixing hot eases 1with the sludge before a mitting the mixture to the dryer, operating the drier at a peripheral speed of approximately. 3200 feet per nte for the tips of the drier lheaters, and maint L a temperature of 300 deg. F., in the separater..

the tips of the drier sludge is carried in a 

